In the control of systems having rotating shafts, torque is a fundamental parameter of interest. Accordingly, torque sensors for sensing the torque applied to a rotating shaft have been developed. Torque sensors that do not contact the rotating shaft are desirable. Non-contact torque sensors may create and/or monitor a magnetic field about a rotating shaft that is proportional to the torque applied to the shaft.
In one known configuration, the rotating shaft, or one or more axial portions thereof, may be circumferentially magnetized, i.e., magnetically polarized in direction following a circular path coaxial with the axis of the shaft. When the shaft is under zero torque, the magnetic field established by the shaft does not include a component in the direction of the shaft axis. When torque is applied to the shaft, the circumferential magnetic field becomes increasingly helical with increasing torque. The helical magnetization resulting from the applied torque includes an axial component in the direction of the axis of the shaft and a circumferential component. The axial component may be directly proportional to the applied torque. A magnetic sensor located proximate the shaft may detect the axial component and provide an output indicative of the level of torque applied to the shaft.
Proper magnetization of the shaft in such an embodiment is critical to operation. Manufacturing errors in magnetizing the shaft or in assembling the shaft in the system, can lead to a complete failure of the sensor system. Also, the shaft magnetization may weaken over time. However, the lack of a sensible axial field component when the shaft is at zero torque does not allow a simple, passive external means for determining if the shaft is magnetized properly or even magnetized at all.
One known approach to providing a sensor having a quiescent axial magnetic field at zero torque involves circumferentially magnetizing a shaft while the shaft is under a predetermined torque. Since the circumferential field is induced in the presence of the predetermined torque, a non-measurable axial field component occurs at the torque applied during magnetization, and when the torque on the shaft is removed the circumferential field is skewed resulting in a quiescent axial field component. Although such a configuration allows diagnostics at zero torque, it also suffers from several deficiencies. For example, magnetizing the shaft under a predetermined torque is a cumbersome and expensive process that does not allow facile large volume production.
Accordingly, there is a need for a torque sensor system including a magnetized shaft providing a measurable axial field component at zero torque that may be efficiently and cost-effectively produced.
Although the following Detailed Description will proceed with reference being made to illustrative embodiments, many alternatives, modifications, and variations thereof will be apparent to those skilled in the art. Accordingly, it is intended that the claimed subject matter be viewed broadly.